Blogger and Google Reader developer Mihai Parparita details how to expand Gmail's one-line "Has the words" input field to a full textarea so that you can more easily build complex and powerful Gmail filters. His method requires editing Firefox's userContent.css file and adding the following:

Firefox's userContent.css file allows you to change and customize display rules for web pages. If you haven't created the file, it may not already exist, so you'll need to create it yourself. To do so, you need to open your profile folder. Windows users, hit Start -> Run and type %appdata%\Mozilla\Firefox\Profiles and head into your current profile (if you have only one that ends with default, that's the folder). Mac users can find it at ~/Library/Application Support/Firefox.

Now that you're in the profile folder, go to the folder called chrome. Rename the userContent-example.css file to userContent.css, open it in your favorite text editor, and paste the code above. The larger textarea does bork the Gmail interface a bit when you click the Create a filter link, but it's a completely acceptable tradeoff; creating complex filters in Gmail just got a helluva lot more readable.